The Physics of the Sublunar RegionChange and Motion

It is frequent to have the impression of Aristotle's cosmos
as being static. It is easy to see where this comes from, in the
Aristotelian sublunar realm, natural motion ceases when the moving
object reaches its natural place, and violent motion comes to an
end when the external force no longer acts. If we put everything in
its rightful place and get rid of external movers, Aristotle's
world will screech to a halt. However, this impression comes from
restricting our attention to one kind of change -- change of place
or "local motion." Look beneath the surface, not at the
location of an object, but at the nature of the object, and the
true dynamism of the Aristotelian cosmos becomes apparent. For
Aristotle, natural things are always in a state of flux; it is part
of their essential nature to be in transition from potentiality to
actuality. This is no doubt most obvious in the biological realm,
where growth and development are inescapable, but Aristotle's
biological studies powerfully shaped his entire philosophy of
nature. His definition of nature, as the inner source of change
found in all natural bodies, may well have had biological origins,
but it was applicable to both the organic and the inorganic realms.
The central object of study in Aristotle's natural philosophy,
the, was change in all of its forms, and manifestations. Aristotle
stated bluntly in his Physics (book 3) that if we are
ignorant of change, we are ignorant of nature. If the gross objects
that fill Aristotelian cosmos seem to prefer rest over motion,
beneath the surface they are seething with change.

Aristotle and his medieval followers identified four kinds of
change:

generation and corruption,

alteration,

augmentation and diminution, and

local motion.

Generation and
corruption occur when individual things (that is, substances) come
into existence and go out of existence. Alteration is change of
quality, as when the cold object becomes warm. Augmentation and
diminution refer to quantitative change -- that is, change of size,
as in rarefaction and condensation. And local motion is change of
place the kind of change that seventeenth-century scientists
elevated to a place of centrality that it did not have within
Aristotelian physics.